Orthogonal Frequency Division based Multiple Access (OFDMA) and its versions like SC-FDMA is finding its way in to many of the modern high data rate communication standards like the IEEE 802.16e/m, IEEE 802.22 and also 3GPP-LTE and LTE-Advanced as the preferred multiple access scheme.
Channel estimation in the uplink in such systems is a challenging task as the received time domain signal is the superimposition of the signals from the multiple users. From this composite signal it is required to estimate the channel coefficients between each pair of transmitter and receiver. Conventionally this is done by making use of pilots. These methods give good performance at the cost of a significant loss in bandwidth efficiency of the system. This loss is severe in a time varying frequency selective fading channel in uplink OFDMA scenarios as the number of users increase. This is because, the number of pilots required to identify all the user channels each with many impulse response coefficients is very high. Hence there is a strong need for bandwidth efficient channel estimation in such systems with acceptable performance.
In recent times, superimposed training (ST) based techniques have been tried out for channel estimation in single carrier, OFDMA and multiple input multiple output (MIMO) systems. In these schemes, training symbols known to the receiver are algebraically added on to the data at a low power, thus avoiding the need for additional time slots for training. At the receiver these known symbols, in the presence of unknown data and noise, are used for channel estimation. These methods for channel estimation are highly attractive compared to pilot assisted techniques as they are bandwidth efficient.
In view of the above reasons ST based methods are attractive for channel estimation. Moreover, the existing literature does not report on any ST based method suitable for application in uplink OFDMA systems so far. Also, there is a strong need for arriving at optimal ST sequences in these systems also.
[Provide brief description of the prior art literatures sl. 1-5 as mentioned in the first technical input, highlighting the disadvantage or limitations of each of the related art vis-à-vis the distinguishing features of the present invention]
In “Channel Estimation for OFDMA Uplink: a Hybrid of Linear and BEM Interpolation Approach,” IEEE Transactions on Signal Processing, vol. 55, no. 4, pp. 1568-1573, April 2007 by Yi Ma, and Tafazolli, R., the channel for each subband is modeled by combing the linear and Fourier basis-expansion functions. Based on this mode, a two-dimensional interpolation approach is proposed for the channel estimation, which is investigated in terms of estimation error and pilot placement. This is a pilot based method and is bandwidth inefficient and is computationally expensive. Moreover, no description is provided on the optimality of the sequences.
“Maximum Likelihood synchronization and channel estimation for OFDMA uplink transmissions,” IEEE Transactions on Communications, vol. 54, no. 4, pp. 726-736, April 2006 by Pun M. O, Morelli M, and Kuo C. J, discloses an alternating projection based method to solve the carrier frequency synchronization, timing synchronization and channel estimation problem as an alternative to the computationally complex search based maximum likelihood solution.
“Asymptotically efficient reduced complexity frequency offset and channel estimators for uplink MIMO-OFDMA systems,” IEEE Trans. on Signal Processing, vol. 56, no. 3, pp. 964-973, March 2008 by S. Sezginer, and P. Bianchi, addresses the joint data-aided estimation of frequency offsets and channel coefficients in uplink MIMO-OFDMA systems. The method again is based on time multiplexed training and hence is bandwidth inefficient.
“Parametric Channel Estimation for Pseudo-Random Tile-Allocation in Uplink OFDMA,” IEEE Transactions on Signal Processing, vol. 55, no. 11, pp. 5370-5381, November 2007 by Raghavendra M. R., Lior, E., Bhashyam, S., and Giridhar, K propose a parametric channel estimation method applicable to irregular and sparsely spaced pilots, that does not exhibit an error-_floor over the nominal operating range of signal to noise ratios, even for highly selective channels. The proposed algorithm exploits the pilot structure in each tile in estimating the delay subspace corresponding to the parametric channel description.
“Line search based iterative joint estimation of channels and frequency offsets for uplink OFDMA systems,” IEEE Trans on Wireless Communications, vol. 6, no. 12, pp. 4374-4388, December 2007 by Y. Na, and H. Minn proposes a novel iterative joint estimation of channels and frequency offsets for uplink OFDMA systems. The proposed method is based on the iterative line search algorithm with a trust region. The objective function minimizes the mean-square distance between the received vector and the reconstructed received vector based on the estimated parameters.
S. Zhou, G. B. Giannakis, and A. Scaglione, “Long codes for generalized FH-OFDMA through unknown multipath channels,” IEEE Trans. on Communications, vol. 49, no. 4, pp. 721-730, April 2001 discloses a host of blind channel estimation algorithms are developed trading off complexity with performance. However the methods suffer from the problems of slow convergence and bandwidth inefficiency.